A method and an assembly is disclosed herein for processing cellulose pulp of wood processing industry. Especially, the disclosed method and assembly may be used, on the one hand, as a pressure diffuser for washing the pulp before an oxygen stage and, on the other hand, for the separation of soap, gas and/or other light materials from filtrates and black liquor of a cellulose mill in a pressurized device, as well as the process connections enabled by the method. The disclosed method and assembly are suitable for processing filtrates from brown stock washing and bleaching as well as black liquors from a digesting department and an evaporation plant. For example, the filtrate of the washing department pumped to the digesting department of a sulfate cellulose mill as well as the black liquor taken from the digesting department to an evaporation plant.
Extractives and resin present in wood react in the sulfate cellulose digesting process with alkaline digesting chemicals and form various compounds generally referred to as soap. The amount of soap when digesting softwood pulp is typically approximately 20 to 80 kg per a ton of pulp. When digesting hardwood pulp, the amount of soap obtained is considerably smaller. In solutions of the prior art, soap is washed away from the pulp in a digester and in the brown stock washing and is thereafter carried along with black liquor to an evaporation plant. At the evaporation plant, soap is separated from black liquor and acidified to tall oil. Tall oil is sold as a raw material for chemical industry or burned. A part of tall oil can be returned to the hardwood digesting to boost the dissolution of extractives from the pulp. Although soap is a useful raw material, when it gets to a wrong place it greatly interferes with various functions of a cellulose mill. If soap separation is not working at the evaporation plant and a significant amount of soap gets into evaporation units along with black liquor, it causes fouling and clogging of evaporators as well as foaming of the liquor whereby the condensates of the evaporation plant are contaminated unusable. In brown stock washing, soap disturbs the flow of washing water through the pulp and significantly weakens the washing efficiency of washers. Also, air is easily mixed with soap which also disturbs the washing of pulp. At the washing department, soap accelerates fouling of apparatus, too. At the digesting department, soap and possibly air along with it may cause malfunction and fouling of apparatus. Due to the several handicaps and for maximizing the yield of soap and tall oil, efficient separation of soap from filtrates and black liquor would be extremely advantageous. In solutions of prior art, soap is separated from filtrates and black liquor in various atmospheric pressure tanks in which soap, that has a lower density than filtrates and liquor, rises up to the surface. At the evaporation plant, soap on the surface in the tank is removed from the tank, for example, by overflow to a separate tank for further processing. If the retention time of filtrate or black liquor is not sufficient or if soap does not separate normally due to other reasons, such as incorrect dry content or alkality of black liquor, considerable amounts of soap are able to disturb the evaporation process. In the filtrate tanks of the washing department, soap is also separated onto the surface of the filtrate in the tank from which it is, in some cases, removed by overflow and taken to the evaporation plant along with black liquor. Usually, soap is not sought to be separated at the washing department area in any way, but it is converted into a more easily dissolving form using various expensive chemicals and washed away to the evaporation plant. If soap is not able to exit properly, it enriches in the filtrate tank and filtrate circulations until a balance is achieved. In cases like this, soap concentrations and process failures may be very large. If the digester wash or the washing of pulp in a digester is successful, most of the soap is directly displaced to black liquor and only small amounts of soap is carried along to the brown stock washing. In most digesters, however, the wash is not sufficient and significant amounts of soap is carried along to the brown stock washing during which it may be enriched to considerably high concentrations. Typical places where such enrichment takes place are the filtrate circulations between the digester and the first brown stock washer.